Dispersion hardened metal product and process



Dec. 8, 1964 F. J. ANDERS, JR 3,159,903

DISPERSION HARDENED METAL PRODUCT AND PROCESS Filed Feb. 26, 1963 ANDERS, JR

FREDERIC J @uaw usages 1 DISPERSEGN HARDENED METAL PRGD'UCT AND PRQCESS Frederic 3'. Anders, Jr Wilmington, DeL, assignor to E. I.

du Pont de Nemours and Company, Wilmington, Bah, a corporationof Delaware p Filed Feb. 26, M63, ar. No. 261,184 7 15 Claims; (Cl. 2f9l82.5)

This invention relates to processes for increasing the high-temperature strength of metals and to the metal compositions having improved properties so produced.

More particularly the invention is directed to processes for increasing the ultimate tensile, yield and stress-rupture strengths of a metal as measured at 73% of its absolute melting point, the metal having an atomic number of 22 to 75, a melting'point above,1100 CI, and an oxide with afree energy of formation at 27 C. of 30 to 105 kcaLper gram atom of oxygen, which processes include the step comprising effecting a- 40 to 95% reduction in cross-sectional area of abody of said metal by mechanically working it atfatemperature not higher than /2 of the absolute melting point of the metal, the metal in said body-having uniformly dispersed therein from 0.1 to 10 volume percent of. particles, to 1000 millimicrons in average size, of a refractory metal oxide insoluble inthe metaLthe oxide having a free energy of formation, measured at 1000 C., greater than 60 kilocalories per gram atom of oxygen, and is further particularly directed to wrought metal compositions, producibleby said processes, the. compositions being characterized by having high ultimate tensile and yield strength as measured at 73% of. their absolute melting points and comprising (1 a metal having a melting point above J1100 C. andan oriented crystal structure, said orientation being irreversible at temperatures below 85% of the absolute melting point ofthe metal, and (2) uniformly dispersed in said metal, from 0.1 to volume percent of particles, 5 to 1000 millimicrons in average size, of a refractory metal oxide insoluble in the metal, the oxide having a free energy of formation, measured at 1000 C greater than 60 kilocalories' per gram atom of oxygen.

The drawing is an electron micrograph showing the metallurgical microstructure of a composition of the invention, said composition containing 2% by volume of thoria in the form of particles 1 dispersed in a matrix of nickel metal, 2, the nickel crystallographic pattern showing orientation of supergrains into-planes 3, 4 and 5, the micrograph having been prepared by a conventional 2- stage, Formvar-carbon metallographic technique.

When metal is cold-worked according to conventional techniques, as by rolling,'forging or swaging, substantial increases in the strength of the metal can be achieved. A several-fold increase is not uncommon. The increase is accompanied by an increase in hardness and loss of ductility, however, and this is often undesirable. The worked piece can be restored to its original hardness and ductility by annealing, but duringannealing the increase in strength attributable to the cold-Working is lost. Similarly, the incremental strength gained by cold-working can be lost if one attemptsto use the cold-worked piece at very high service temperatures, such use being the equivalent of annealing. v V

' It has been suggested thatwhen mixtures of (1) refractory oxide powder particles and 2) powders of metals,

? aisaass Patented Dec. 8, 1964 such as iron and cobalt, which undergo phase transformation at elevated temperatures, are compacted and then hot-deformed at an elevated temperature not exceeding the phase transformation temperature, the dissipation of a stored energy from the product is greatly inhibited; This theoretical statement of the result obtained necessarily implies that upon heating such systems at or above the phase transformation temperature, as when the products are annealed or subjected to service above the phase transformation temperature, the stored enrgy is dissipated, and it follows that any beneficial effects of such energy storage are thereupon lost.,

ties substantially persist even after the products are annealed. It is clear that the embedment of the refractory oxide dispersoid within the metal is responsible for this unique and unexpected result, probably because such embedment prevents association of the refractory oxide particles during compaction and avoids stringering in l the worked product-that is, a displacement of the refractory oxide as stringers, interspersed with regions of metal unmodified with oxide. V

More" particularly, it has now been found that if a metal having an atomic number. of22 to 74; a melting point above 1100 C., and an oxide with a free energy of formation at 27 C. of 30 to-105 kcal. per gram atom of oxygen, is first modified by a uniform dispersion therein ofrefractory metal oxide particles within certain critical limits, and when'so modified a body of said metal is mechanically' worked at a temperature not higher than /2 i of the absolute melting point of the metal, until a 40 to 95 reduction in its cross-sectional area has beeneffected the ultimate tensile, yield, and'str'es's-rupture strengths of the metal, as measured at 73% of it's absolute melting point, is substantially increased, and this increase is retained even after annealing or after extended use; at very hightemperaturesj The wrought metal compositions so obtained are characterized by havingan oriented crystal structure of the metal, and this orientation is irreversible at temperatures below ofthe absolute melting point of the metal. 7 r

Metals which have the required atomic number, melting point, and oxide free energy for use in this invention include titanium, vanadium, chromium, manganese, iron, cobalt, nickel, columhium', molybdenum, masurium, tan talum, and tungsten. These metals may be alloyed with each other and with minor proportions of other elements, provided the melting point of the alloy, without the dis-, persoid refractory oxide present, is above the specified 1lC-0 C. The total of such other elements will in no event exceed half of the total'weight of the composition without dispersoid.

The refractory metal oxide dispersed in the metal, herein sometimes called the dispersoid, must have a-free energy of formation at 1000" C. greater than 60"kilocalories'per gram atom of oxygen. Mixtures of such oxides,

and double oxides having the characteristics of the single oxides, can be used. For products to be used at highest service temperatures, oxides of highest free energies of formation are preferred. A typical group of suitable oxides, and their free energies of formation (AF) is shown in the following table:

The refractory oxide particles must be in the size range of to 1000 millimicrons (mg) in average dimension. This means that, at the bottom of the range, oxide particles of molecular size will not suffice. A particle size of 5 to 250 m, is preferred, with a lower limit of m nd an upper limit of 150 m being especially preferred. It will be recognized that the preferred sizes are within the range of colloidal dimensions, and methods already known for preparing colloidal particles can be used to make suitable refractory oxide particles.

The proportion of refractory oxide dispersoid particles in the metal must be in the range from 0.1 to 10 percent by volume, a range of 0.5 to 5% being preferred. Especially excellent results have been achieved with a dispersoid proportion of 2% by volume.

It is essential that a plurality of the refractory oxide particles be uniformly dispersed in the metal. The most effective method to achieve such dispersion is by coprecipitating the hydrous oxide of the metal and the dispersoid particles, reducing the metal oxide to metal with hydrogen, and sintering the reduced product, all as described in Alexander et al. United States Patent 3,019,103. For metals difficult or impossible to reduce with hydrogen, such as titanium and columbium, reduction of the coprecipitated hydrous metal oxide containing the dispersoid can be carried out in a fused salt bath, using an active metal such as sodium or calcium as the reducing agent, all as more fully described in US. patent application Serial No. 27,967, filed May 9, 1960, by Alexander and Yates. While ordinary mechanical blending of powdered metal and powdered dispersoid is inadequate to give a useable mixture, prolonged ball-milling of such powder blends, as for upwards of 30 hours, under conditions causing the dispersoid to become embedded within the metal gives a mixture which can be further processed according to the invention.

Having prepared a uniform dispersion of refractory oxide dispersoid particles in a specified metal as abovedescribed, the mixture is compacted and sintered, if necessary, to give a shaped body suitable as the work-piece to receive the mechanical working of the present invention. Methods of effecting compacting and sintering are well known, and any such method can be used.

To compact the powder products obtained in some of the above-disclosed methods of embedment the powder can, for example, be loaded into a rubber boot and the boot hydraulically compressed. Alternatively, die-pressing or powder-rolling or the like can be used. Such compaction should density the product to upwards of 70% of theoretical density.

The sintering of the compact effects a number of de sirable changes in the compact, especially when carried out in hydrogen at elevated temperatures. The last traces of the reducible metal oxide are reduced. The strength of the compact increases because the metal surfaces are then free of oxide and can bond together more readily. The density of the metal product increases; the porosity 4 correspondingly decreases. The specific surface area of the compact decreases because of the elimination of the pores, so that reoxidation of the product upon exposure to air is much less severe or even non-existent. The sintered product may have a surface area as low as, sa .01 square meter per gram.

After sintering, the refractory oxide-modified metal can be hot-worked to eliminate essentially all of the porosity remaining in the work-piece. This working is carried out at elevated temperaturesthat is, above the recrystallization temperature of the metal as measured without dispersoid present. For instance, for nickel one would use temperatures above 850 F. (500 C.). The working can be accomplished in various ways, such as by extrusion, forging, swaging, rolling, or other, like methods.

Since the object of the hot-working is to bring up the density of the product to 100% of theoretical, the workpiece is subiected to a sufiicient working to insure this result. One very practical way of doing this is by hotextrusion. This squeezes the metal to eliminate voids and at the same time efiects a reduction in cross-sectional area which results from a slippage or plastic flow of the material as it goes through the extrusion die. This action is well understood in the art.

The suitably prepared work-piece, whether prepared by the above-described procedures or by variations or modifications thereof, comprises a uniform dispersion of the dispersoid refractory oxide particles in the matrix metal, the system having a density which is substantially 100% of theoretical. This material is now ready for mechanical working according to a process of the invention.

The work-piece is now mechanically worked at a temperature no higher than /2 of the absolute melting point of the metal, the melting point being as measured without the dispersoid present. The absolute melting point means the melting point expressed in degrees absolute. The melting points of the metals involved are either already available in tables of critical constants, or, as in the case of alloys, can be readily determined experimentally according to known procedures. The temperature of working is critical and must be in the range stated if the full benefits of the invention are to be realized.

It is important to realize that in metal-working temperatures are often given as the temperature of the workpiece as it is initially subjected to the working operation. The temperature is often not actually measured during the deformation process. it is very difficult, for example, to take the temperature of a billet within an extrusion press, so the usual practice is to measure the temperature of the billet as it is fed into the press and call this the temperature of working. Though not very precise, the practice is meaningful as long as it is consistently followed.

It is also critically important that the extent of working be that which effects a 40 to reduction in crosssectional area of the work-piece. Less than 40% reduction does not provide sufiicient working to enhance the properties to the desired degree. More than 95% reduction renders the piece over-worked-that is, the piece often exhibits a loss of the enhanced properties when exposed to very high temperatures. When the extent of cross-section reduction is to be at or near the maximum of the specified range it is often preferred to work the piece only part waysay, to 60% reduction, and then anneal the worked piece and thereafter work the piece to its ultimate reduction.

The mechanical working can be accomplished by such procedures as extrusion, rolling, forging, swaging, drawing, or similar procedures known to the art.

The unique result obtained according to the present invention is that when a metal product is worked according to the processes just described, it can be annealed to restore it to its original hardness and ductility but during the annealing process the increase in strength measured,

two-inch diameter billet.

. e3 at 73% of the absolute melting point, attributable to the working, is not lost. 1 V

The term annealing as used herein refers to a hightemperature treatment which produces softening of the metal at room temperature and, usually, an improvement in ductility. Annealing consists ingiving a metal a treatment such that its room temperature hardness decreases and its ductility increases. Annealing is abroad term and it may accomplish the stated results in various manners. Thus, the heatingmay be to such a high temperature that recrystallization of the metal actually occurs. On the other hand, it is not necessary that recrystallization occur in order to get the results stated. It a soluble phase and another phase are present, the heating may be only enough to cause some of the soluble metal phase to dissolve in the matrix phase. This is known as a solution heat-treat ment, and it may anneal the metal depending on the cooling rate, etc. Again, the heating may be to such a high temperature that the phase transformation temperature is exceeded.

Processes of the present invention wherein the worked product is subsequently annealed at a temperature which is from 65 to 95% of the melting point, in degrees absolute, of the metal unmodified with the dispersoid represent a preferred embodiment. The products from such processes have substantially the same ductility and hardness V as before the working was effected, but retain substa'ntially allof the enhanced properties attributable to the Working.

The novel products of the invention, produced by the processes above-described, are wrought metal compositions having high ultimate tensile, yield, and stress-run ture strengths, as measured at 73% of their absolute melting points, as compared with similar products in invention to an outstanding degree.

As will be seen from the drawing, the oriented crystal structure in the products of the invention is observable upon metallographic examination as colonies of adjacent grains in the form of super-grains, with the dispe-rsoid. particles uniformly dispersed therethrough. Such metallographic examination can be made according to techniques commonly used in the art, the following being representative. V

Samples are mounted in Bakelite and polished in sequence through 4/ paper. They are removed from the mount and thermally etched for five hours at 1000 C.

, in a high vacuum (*5 mm. Hg). A thin layer of carbon is vacuum-deposited on the metal surface. The

metal is dissolved away with a 12% bromine solution. The carbon replica is mounted on a 250-mesh screen and viewedwith an electron microscope.

The invention Will be better understood by reference to the following illustrative examples.

Example 1 .Nickel powder containing 2% by volume of a dispersed phase of submicron thorium oxide particles, prepared by a codeposition process of Alexander et al., US.

Patent 3,019,103, was hydrostatically compacted into a This billet was then sintered at. 2200 F. for six hours in dry hydrogen and extruded to a -inch diameter rod at 1700 F. The tensile properties at 1800 F. of the as-extruded rod were:

Ultimate tensile strength, p.s.i 5,900

Yield strength at 0.2% offset, p.s.i 4,900

. Stress-rupture at 9000 p.s.i., hours Elongation, percent tin-l1 12 Reduction in cross-sectional area, percent 26.6

The rod wasthen swaged at room temperature to 72% Y in cross-sectional area and again tested in tension at 73 Ultimate tensile strength, p.s.i, 16,750 Yield strength at 0.2% otrset, p.s.i 15,900 Elongation, percent 6 Reduction in cross-sectional area, percent 16.9

Example 2 I 7 Another sample of the swaged rod of Example 1 was annealed by heating it in air for 3 hours at 2200 F. and again measuring the tensile properties of the annealed sample at 1800" F. Resultswere as follows:

Ultimate tensile strength, p.s.i 1 16,900 Yield strength at 2% ofiset, p.s.i.- 16,100 Elongation, percent i 6 Reduction in cross-sectional area, percent 22.5 Stress-rupture at 900 p.s.i. (annealed at 2400 F), hours -i 20.2

The powder was pressed at room temperature to form a a slab 0.65" thick by a total force of 360 tons'acting on the die punches. The slab was sintered at 2200 F. for 8 hours in dry hydrogen, and then densified to substantially 100% of theoretical density by hot-rolling at 2200 F. The surfaces of the hot-rolled product were machined to leave a. conditioned plate having a thickness of 0.160 inch.

In six roll-passes through the rolling mill at room temperature the conditioned plate was now reduced in thickness from .16 to .04 inch. The product Was then examined for tensile properties at 1800 F. The tensile strength was found to be 13,300 p.s.i.

Another sample of the .04 inch-thick sheet was annealed for 1 hour at 2000 F. in dry hydrogen and tensiletested at 1800 F. The tensile strength was found to be 14,000 p.s.i.

-- Iclaim:

1. In a process for increasing the ultimate tensile, yield, and stress-rupture strengths of a metal, as measured at 73 of its absolute melting point, the step comprising effecting a 40 to reduction in cross-sectional area of a body of said metal, having a density substantially of theoretical, by mechanically working it at a temperature not higher than /2 of the absolute melting point of the metal, the metal in said body having an atomic number of 22 to74, a melting point above 1100 C., and an oxide with a free energy of formation at 27 C. of from 30 to kcal. per gram atom of oxygen in the oxide, said metal having uniformly dispersed therein from 0.1

to 10 volume percent of particles, 5 to 1000 millimicrons in average size, of a refractory metal oxide insoluble in themetal, the oxide having a free energy of formation, measured at 1000 C., greater than 60 kilocalories per gram atom ofoxygen.

2. A process of claim 1 wherein the metal is nickel. 3. A process of claiml wherein the refractory oxide is thoria.

4, A process of claim 1 wherein the metal is nickel 7 ture which is from 65 to 95% of the absolute melting point of the metal unmodified with the refractory oxide.

6. A wrought metal composition characterized by having a density substantially 100% of theoretical and having high ultimate tensile, yield, and stress-rupture strengths as measured at 73% of its absolute melting point, the composition comprising (1) a metal having an atomic number of 22 to 74, a melting point above 1100" C., and an oxide with a free energy of formation at 27 C. of from 30 to 105 kcal. per gram atom of oxygen in the oxide, said metal also having an oriented crystal structure,- the orientation being irreversible at temperatures below 85% of the absolute melting point of the metal, and (2) uniformly dispersed in said metal, from 0.1 to 10 volume percent of particles, 5 to 1000 millimicrons in average size, of a refractory metal oxide insoluble in the metal, the oxide having a free energy of formation, measured at 1000 C., greater than 60 kilocalories per gram atom of oxygen.

7. A composition of claim 6 in which the metal is nickel.

'8. A composition of claim 6 in which the refractory oxide is thoria.

9. A composition of claim 6 wherein the metal is nickel and the refractory oxide is thoria.

10. A wrought, annealed metal composition characterized by having a density substantially 100% of theoretical and having high ultimate tensile, yield, and stressrupture strengths as measured at 73% of its absolute melting point, the composition comprising (1) a metal having an atomic number of 22 to 74, a melting point above 1100 C., and an oxide with a free energy of formation at 27 C. of from 30 to 105 kcal. per gram atom of oxygen in the oxide, said metal also having an oriented crystal structure, the orientation, being irreversible at temperatures below 85% of the absolute melting point of the metal and being observable upon metallographic examination as colonies of adjacent grains in the form of super-grains, and (2) uniformly dispersed in said metal, from 0.1 to volume percent of particles, 5 to 1000 millimicrons in average size, of a refractory metal oxide insoluble in the metal, the oxide having a free energy of 8 formation, measured at 1000 C., greater than. kilocalories per gram atom of oxygen.

11. A composition of claim 10 in which the metal is nickel.

12. A composition of claim 10 in which the refractory oxide is thoria.

13. A composition of claim 10 in which the metal is nickel and the refractory oxide is thoria.

14. In a process for producing metal bar comprising nickel. having substantially uniformly dispersed therein submicron-size thoria particles as a strengthening material the steps comprising (1) compacting into a billet a nickel powder having about 2% by volume of submicron thoiia particles dispersed therein and consolidating said billet to a body having a density substantially 100% of theoretical, and (2) cold working said body by swaging it to a reduction in cross-sectional area of about from 40 to whereby a bar product having a tensile strength of at least 16,000 p.s.i. at 1800 F. is produced.

15. A swaged metal bar comprising nickel having a density substantially of theoretical and having substantially uniformly dispersed therein about 2% by volume of submicron-size thoria particles, said nickel having an oriented crystal structure, the orientation being irreversible at temperatures below 85% of the absolute melting point of the metal, said bar product having a tensile strength of at least 16,000 p.s.i. at 1800 F.

References Cited in the file of this patent Journal of Metals, vol. 14, No. 8, August, 1962, pp. 561562. 

1. IN A PROCESS FOR INCREASING THE ULTIMATE TENSILE, YIELD, AND STRESS-RUPTURE STRENGTHS OF A METAL, AS MEASURE AT 73% OF ITS ABSOLUTE MELTING POINT, THE STEP COMPRISING EFFECTING A 40 TO 95% REDUCTION IN A CROSS-SECTIONAL AREA OF A BODY OF SAID METAL, HAVING A DENSITY SUBSTANTIALLY 100% OF THEORETICAL, BY MECHANICALLY WORKING IT AT A TEMPERATURE NOT HIGHER THAN 1/2 OF THE ABSOLUTE MELTING POINT OF THE METAL, THE METAL IN SAID BODY HAVING AN ATOMIC NUMBER OF 22 TO 74, A MELTING POINT ABOVE 1100*C., AND AN OXIDE WITH A FREE ENERGY OF FORMATION AT 27*C. OF FROM 30 TO 105 KCAL. PER GRAM ATOMS OF OXYGEN IN THE OXIDE, SAID METAL HAVING UNIFORMLY DISPERSED THEREIN FROM 0.1 TO 10 VOLUME PERCENT OF APRTICLES, 5 TO 1000 MILLIMICRONS IN AVERAGE SIZE, OF A REFRACTORY METAL OXIDE INSOLUBLE IN THE METAL, THE OXIDE HAVING A FREE ENERGY OF FROMATION, MEASURED AT 1000*C., GREATER THAN 60 KILOCALORIES PER GRAM ATOM OF OXYGEN.
 6. A WROUGHT METAL COMPOSITION CHRACTERIZED BY HAVING A DENSITY SUBSTANTIALLY 100% OF THEORECITCAL AND HAVING HIGH ULTIMATE TENSILE, YIELD, AND STRESS-RUPTURE STRENGTHS AS MEASURED AT 73% OF ITS ABSOLUTE MELTING POINT, THE COMPOSITION COMPRISING (1) A METAL HAVING AN ATOMIC NUMBER OF 2I TO 74, A MELTING POINT ABOVE 1100*C., AND AN OXIDE WITH A FREE ENERGY OF FROMATION AT 27*C. OF FROM 30 TO 105 KCAL. PER GRAM ATOM OF OXYGEN IN THE OXIDE, SAID METAL ALSO HAVING ANORIENTED CRYSTAL STRUCTURE, THE ORIENTATION BEING IRREVERSIBLE AT TEMPERATURES BELOW 85% OF THE ABSOLUTE MELTING POINT OF THE METAL, AND (2) UNIFORMLY DISPERSED IN SAID METAL, FROM 0.1 TO 10 VOLUME PERCENT OF PARTICLES, 5 TO 1000 MILLIMICRONS IN AVERAGE SIZE, OF A REFRACTORY METAL OXIDE INSOLUBLE IN THE METAL, THE OXIDE HAVING A FREE ENERGY OF FORMATION, MEASURED AT 1000*C., GREATER THAN 60 KILOCALORIES PER GRAM ATOM OF OXYGEN. 